Encapsulated emissive element for fluidic assembly

ABSTRACT

A method is provided for fabricating an encapsulated emissive element. Beginning with a growth substrate, a plurality of emissive elements is formed. The growth substrate top surface is conformally coated with an encapsulation material. The encapsulation material may be photoresist, a polymer, a light reflective material, or a light absorbing material. The encapsulant is patterned to form fluidic assembly keys having a profile differing from the emissive element profiles. In one aspect, prior to separating the emissive elements from the handling substrate, a fluidic assembly keel or post is formed on each emissive element bottom surface. In one variation, the emissive elements have a horizontal profile. The fluidic assembly key has horizontal profile differing from the emissive element horizontal profile useful in selectively depositing different types of emissive elements during fluidic assembly. In another aspect, the emissive elements and fluidic assembly keys have differing vertical profiles useful in preventing detrapment.

RELATED APPLICATIONS

Any and all applications, if any, for which a foreign or domesticpriority claim is identified in the Application Data Sheet of thepresent application are hereby incorporated by reference under 37 CFR1.57.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention generally relates to visual display technology and, moreparticularly, to a visual display substrate made from partiallyencapsulated emissive elements developed for fluidic assembly.

2. Description of the Related Art

A color display is composed of pixels that emit light at threewavelengths corresponding to the visible colors red (R), green (G), andblue (B), which is referred to as an RGB display. The RGB components ofthe pixel are turned on and off in a systematic way to additivelyproduce the colors of the visible spectrum. There are several displaytypes that produce the RGB images in different ways. Liquid crystaldisplays (LCD) are the most prevalent technology and they produce RGBimages by shining a white light source, typically a phosphor producedwhite light emitting diode (LED), through a color filter of a subpixel.Some portion of the white light wavelengths is absorbed and sometransmitted through the color filter. The weaknesses of LCD, which thecurrent disclosure directly addresses, are 1) low efficiency where onlyabout 5% of the light generated by the backlight is seen as an image bythe user, and 2) low dynamic range because the LC material cannotcompletely block light to produce a black pixel.

Organic light emitting diode (OLED) displays produce RGB light by directemission of each of those wavelengths of light at a pixel level withinthe organic light emitting material. The weaknesses of the OLED displayare poor reliability and low efficiency (˜5% QE) of the blue OLEDmaterial.

A third display technology is the micro-LED display. This displaytechnology uses micro-sized (10 to 150 μm diameter) inorganic LEDs fordirect emission of light at the pixel level. In order to make an RGBdisplay using micro-LEDs it is necessary to assemble large area arraysof three different micro-LEDs that emit in each of the RGB range ofwavelengths. The low-cost manufacture of micro-LED displays requires theuse of a massively parallel assembly technique to position millions ofindividual micro-LEDs in regular arrays.

As used herein, a micro-LED (uLED) is an LED with a diameter orcross-sectional area of 100 microns or less. Both organic and inorganicLED displays have a very high contrast because black pixels are set toemit no light. For an inorganic uLED display, blue gallium nitride (GaN)LEDs have a 35-40% quantum efficiency, with a reliability of over 50,000hours, as has been established in general lighting. Sony has developed apassive matrix of uLEDs arranged in a display array using apick-and-place system. However, since large displays require millions ofLEDs, displays made by this process are time and cost prohibitivecompared to other technologies. Mass transfer technologies use finelypitched stamps to deposit multiple LEDs simultaneously, but this methodis only practical for small area devices (e.g., a watch) because thespacing between LEDs is fixed by the growth substrate.

The fluidic transfer of microfabricated electronic devices,optoelectronic devices, and sub-systems from a donor substrate/wafer toa large area and/or unconventional substrate provides a new opportunityto extend the application range of electronic and optoelectronicdevices. For example, display pixel size LED micro structures, such asrods, fins or disks, can be first fabricated on small size wafers andthen be transferred to a large area glass substrate to make a directemitting display requiring no backlighting. An example of thistechnology is presented in U.S. Pat. No. 9,825,202 and Ser. No.15/412,731, which are incorporated herein by reference.

FIG. 1 is an artist's rending of an optical micrograph of triangulargallium nitride micro-LEDs with a 40 micron (μm) base fractured duringlaser-liftoff. High speed assembly is necessarily higher energy, and asa consequence of the stochastic nature of massively parallel fluidicassembly, component collisions are numerous, difficult to control, andcan easily damage unprotected microcomponents, especially along thegallium nitride crystal planes. Such damage may deactivate themicrocomponent, resulting in an unusable assembled substrate, but thereis also a significant risk that the damage results in fractured piecesthat spread and cause defects in a plurality of sites. If a smallparticle is deposited in a trap site before a micro-LED is assembled,then the particle can prevent the micro-LED from contacting theelectrodes in the bottom of the trap site. It is therefore critical thatmicrocomponents used for fluidic assembly are structurally robust enoughto undergo the many collisions in fluidic assembly without damage.Micro-LEDs in fluidic assembly are particularly susceptible to fractureas epitaxial layers are generally thin (i.e., under 10 microns), whilethe emitter width is generally greater than 20 microns. Also, certainmaterial systems, such as gallium arsenide (GaAs) are particularlysusceptible to fracture. Non-circular shapes, such as shown in FIG. 1are also more easily fractured.

Further, a major challenge for manufacturing emissive displays viafluidic assembly is the rapid and high yield assembly of three types ofemitter such as red-emitting microcomponents, green-emittingmicrocomponents, and blue-emitting microcomponents. A particularchallenge for multi-emitter assembly is simultaneously optimizing theassembly characteristics as well as the emitter characteristics that arenecessary for the production of a high-quality display, while preventingdefects from cross-contamination and component damage.

It would be advantageous if micro-LEDs could be packaged in a protectivecoating during assembly to prevent damage.

It would also be advantageous if the packaged micro-LEDs could be shapedto promote self-assembly in targeted regions of a substrate.

It would be even more advantageous if shape-packaged micro-LEDs could beused during fluidic assembly to coordinate the alignment of particularLED colors into specific subpixel positions on a display substrate.

SUMMARY OF THE INVENTION

Disclosed herein is an emissive substrate made from fluidicallyassembled composite emissive elements, which may be semiconductor-basedinorganic micro-LEDs that are at least partially encapsulated in anothermaterial, such as a patternable polymer (e.g., SU8). The result is thedecoupling of fluidic assembly, which is primarily mediated by theinteraction of the polymer encapsulant structure with the fluidicassembly forcing and trapping sites (wells) in the substrate, from theemissive element, which may be processed and structured to optimizeemissive characteristics such as efficiency, color, emission area, andelectrical contacts, but which may have non-optimal fluidic assemblycharacteristics. The emissive element may be an inorganic emitter suchas a gallium nitride (GaN) or gallium arsenide (GaAs)-based micro-LED,with a patternable polymer encapsulant that protects the emitter fromcollisions and subsequent damage during fluidic assembly. Theencapsulation protects the emitting micro-LED and creates greatercontrol over the shapes critical to efficient assembly—both of whichenable higher speed assembly and fewer defective arrays. Theencapsulation material may optionally serve additional functions such aselectrical passivation between micro-LED cathode and anode or lightmanagement by absorbing or reflecting light.

Accordingly, an exemplary method is provided for fabricating anencapsulated emissive element. Beginning with a growth substrate, aplurality of emissive elements is formed. Each emissive element has abottom surface attached to a top surface of the growth substrate, a topsurface, and a profile. The growth substrate top surface is conformallycoated with an encapsulation material. The encapsulation material may bephotoresist, a polymer, a light reflective material, a light absorbingmaterial, or a magnetic material. The encapsulant is patterned to formfluidic assembly keys having a profile differing from the emissiveelement profiles. Simultaneously, at least one contact opening is madeto each emissive element top surface. In the case of a surface mountemissive element, two contact openings are made. Then, a firstelectrical contact is made to each emissive element top surface. In thecase of a surface mount emissive element, two electrical contacts aremade. The emissive element top surfaces are bonded to a handlingsubstrate. Once bonded to the handling substrate, the emissive elementscan be separated from the growth substrate, and finally, the emissiveelements are separated from the handling substrate. In one aspect, priorto separating the emissive elements from the handling substrate, afluidic assembly keel or post is formed on each emissive element bottomsurface. Note: the emissive element top surface is defined as thesurface formed adjacent the growth substrate during fabrication, asdescribed in detail below.

In one variation, the emissive element top surfaces are substantiallyplanar in a horizontal orientation, and they have a horizontal profile.The fluidic assembly key has horizontal profile differing from theemissive element horizontal profile, as defined from a vantageorthogonal to the emissive element top surface. For example, a firstfluidic assembly key horizontal profile may be associated with anemissive element capable of emitting a first wavelength of light. Infact, the method may form a plurality of emissive element types (notnecessarily simultaneously), with each type capable of emitting light ata different wavelength. Then, the fluidic assembly keys can be formed ina plurality of different horizontally oriented shapes, with eachhorizontally oriented shape associated with a corresponding emissiveelement wavelength of light emission. Alternatively, the fluidicassembly keys may be formed in a plurality of different verticallyoriented shapes, with each vertically oriented shape associated with acorresponding emissive element wavelength of light emission. A verticalprofile is defined from the vantage orthogonal to an emissive elementsidewall. For example, if the different emissive element types havediffering thicknesses, the thicknesses of the corresponding fluidicassembly keys can be made to compensate, to produce encapsulatedemissive elements all having a uniform thickness.

In another aspect, the emissive elements have a vertical profile and thefluidic assembly vertical profiles differ from the emissive elementvertical profiles. For example, the fluidic assembly key verticalprofiles may have a slope formed between a fluidic assembly key bottomsurface aligned with the emissive element bottom surface, and a fluidicassembly key top surface aligned with the emissive element top surface.The fluidic assembly key top surface width being greater than or equalto the fluidic assembly key bottom surface width. Also, the emissiveelement top and bottom surfaces may be substantially planar in ahorizontal orientation, with emissive element sidewalls forming avertically oriented slope between an emissive element top surface widthless than or equal to an emissive element bottom surface width.

Additional details of the above-described method, an encapsulatedemissive element device, and a display made using encapsulated emissiveelements are provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an artist's rendering of an optical micrograph of triangulargallium nitride micro-LEDs with a 40 micron (μm) base fractured duringlaser-liftoff.

FIGS. 2A and 2B are partial cross-sectional views of encapsulatedemissive elements and FIGS. 2C and 2D are plan (top) views of theencapsulated emissive elements.

FIGS. 3A through 3D are partial cross-sectional views of encapsulatedemissive element vertical profiles.

FIGS. 4A and 4B are, respectively, partial cross-sectional and planviews of a fluidic assembly emissive display.

FIGS. 5A through 5C are partial cross-sectional views depictingexemplary emissive element and fluidic assembly key sidewall profilescontrasting the component shape to the trap site shape.

FIGS. 6A and 6B are, respectively, partial cross-sectional and plan (topdown) views of a micro-LED with contacts in the vertical chipconfiguration.

FIGS. 7A through 7I are partial cross-sectional views depicting a sampleprocess flow that enables the partial encapsulation of a verticalconfiguration micro-LED in a patterned polymer matrix.

FIGS. 8A through 8J are partial cross-sectional views depicting analternative sample process flow that enables the partial encapsulationof a vertical configuration micro-LED in a patterned polymer matrixusing a thin buffer layer between encapsulant and growth substrate.

FIGS. 9A and 9B are, respectively, partial cross-sectional and planviews showing encapsulation prior to release from a growth substrate.FIGS. 9C and 9D are, respectively, partial cross-sectional and planviews showing encapsulation after release from a growth substrate. FIG.9E is a plan view showing partial encapsulation of the emissive element.

FIGS. 10A through 10I depict an exemplary process flow for the partialencapsulation of surface mount configuration composite microcomponentsprior to release from a growth substrate.

FIGS. 11A through 11H depict an exemplary process flow for the partialencapsulation of surface mount configuration composite microcomponentsafter release from a growth substrate.

FIGS. 12A and 12B are partial cross-sectional views depicting a displaysubstrate before and after the deposition of encapsulated verticalemissive elements.

FIGS. 13A and 13B are partial cross-sectional views depicting a displaysubstrate before and after the deposition of encapsulated surface mountemissive elements.

FIG. 14 is a schematic depicting the relationship between trap siteshapes, encapsulant shapes, and emissive element shapes.

FIGS. 15A and 15B are partial cross-sectional views of a surface mountemissive element showing the use of an encapsulant as an electricalinsulator.

FIGS. 16A through 16C are partial cross-sectional views depicting theuse of an encapsulating material in light management.

FIG. 17 is a flowchart illustrating a method for fabricating anencapsulated emissive element.

DETAILED DESCRIPTION

FIGS. 2A and 2B are partial cross-sectional views of encapsulatedemissive elements and FIGS. 2C and 2D are plan (top) views of theencapsulated emissive elements. An encapsulated emissive element 200comprises an emissive element. Emissive elements 202 a and 202 b areshown. Each emissive element has a profile, a top surface 204, a bottomsurface 206, sidewall surfaces 208 between the top and bottom surfaces,and a pair of electrical contacts 210 and 212. In one aspect, as shownin FIG. 2A, the emissive element 202 a further comprises a keel or post213 extending from the emissive element bottom surface 206.Alternatively but not shown, the emissive element may comprise aplurality keels. Further, although the keel is depicted as acylindrically shaped structure, other shapes are also useful, asdepicted in FIGS. 3B, 3C, and 3D. Note: the emissive element top surfaceis defined as the surface formed adjacent the growth substrate duringfabrication, as described in detail below.

One example of an emissive element is a light emitting diode (LED).Although not explicitly shown here (see FIG. 6A), an inorganic LEDtypically comprises a first semiconductor layer, with either an n-dopantor a p-dopant, a second semiconductor layer with the dopant type notused in the first semiconductor layer, and a multiple quantum well (MQW)layer interposed between the first semiconductor layer and the secondsemiconductor layer. The MQW layer may include a series of quantum welllayers comprising indium gallium nitride (InGaN) and gallium nitride(GaN)) not shown. There may also be an aluminum gallium nitride (AlGaN)electron blocking layer between MQW layers and the p-doped semiconductorlayer. The outer semiconductor layer may be p-doped GaN (Mg doping) toform a blue LED, or a green LED if a higher indium content is used inthe MQW. The most practical first and second semiconductor layermaterials are either gallium nitride, capable of emitting a blue orgreen light, and aluminum gallium indium phosphide (AlGaInP), capable ofemitting red light.

A fluidic assembly key at least partially encapsulates the emissiveelement to form a profile, different than the emissive element profile.Fluidic assembly keys 214 a and 214 b are shown respectively partiallyencapsulating emissive elements 202 a and 202 b. FIGS. 2A and 2C depicta surface mount or flip-chip emissive element 202 a, where firstelectric contact 210 and second electrical contact 212 extend from thetop surface 204, while FIGS. 2B and 2D depict a vertical emissiveelement 202 b, where the first electrical contact 210 extends from thetop surface and the second electrical contact 212 extends from thebottom surface 206. Typically, the electrical contacts are formedexclusively on the top or bottom surfaces without extending over theemissive element sidewalls. The electrical contacts may be a metal,doped semiconductor, or transparent conductive oxide (TCO) such asindium tin oxide (ITO). Although not explicitly shown as a distinctlayer, the electrical contacts 210 and 212 may be solder orsolder-coated (e.g., a eutectic solder) for subsequent connection toelectrodes on a display substrate.

The fluidic assembly key is made from a material that may bephotoresist, a polymer, light reflective, magnetic, or light absorbing.In the case of surface mount emissive element 202 a, the fluidicassembly key 214 a may be an electrical insulator isolating the firstelectrical contact 210 from the second electrical contact 212.

In one aspect, the emissive element top surfaces 204 are substantiallyplanar in a horizontal orientation, parallel to electrode plane 216shown in FIG. 2A. “Substantially planar” is defined herein as at least50% of the top surface being aligned in a common plane that is parallelto an electrode plane 216, which is aligned with the interface surfaceof first electrical contact 210 in the case of a vertical emissiveelement. In the case of a surface mount emissive element, the electrodeplane 216 is aligned with the interface surfaces of both the firstelectrical contact 210 and the second electrical contact 212. Anelectrical contact interface surface is the surface making contact to anelectrical interface on a display substrate, as can be seen in FIG. 4A.As can be seen, the electrode plane is parallel with the substrate topsurface 404. The depicted emissive elements 202 a and 202 b haverespective horizontal profiles 218 a and 218 b, and the fluidic assemblykeys 214 a and 214 b have a differing horizontal respective profiles 220a and 220 b, as defined from a vantage orthogonal to the emissiveelement top surface 204. In FIG. 2C the emissive element horizontalprofile 218 a is square and the fluidic assembly key horizontal profile220 a is circular. In FIG. 2D the emissive element horizontal profile218 b is circular and the fluidic assembly key horizontal profile 220 bis triangular.

In one aspect, the emissive element (e.g., 202 a) is capable of emittinglight having a first wavelength, where the first wavelength isassociated with a first fluidic assembly key horizontal profile 220 a.Further, the fluidic assembly key horizontal profiles 220 a and 220 bmay be one of a plurality of different horizontally oriented shapes,with each horizontally oriented shape associated with a differentwavelength of emissive element light emission. In this example, thecircular fluidic assembly key horizontal profile 214 a may be associatedwith the emission color blue and the triangular fluidic assembly keyhorizontal profile 214 b associated with the emission color red.Likewise, the emissive element horizontal profiles 218 a and 218 b maybe of differing horizontally oriented shapes, with each emissive elementshape associated with a different wavelength of emissive element lightemission. Using the above example, the square emissive elementhorizontal profile 218 a may be associated with the blue emission colorand the circular emissive element horizontal profile 218 b associatedwith the red emission color.

FIGS. 3A through 3D are partial cross-sectional views of encapsulatedemissive element vertical profiles. In one aspect, the emissive elementshave vertical profiles with vertically oriented shapes as defined by athickness between the emissive element top 204 and bottom 206 surfaces,where each emissive element vertical shape is associated with adifferent wavelength of emissive element light emission. Although onlysurface mount emissive elements are depicted, vertical emissiveelements, with an electrical contact on each of the top and bottomsurfaces (see FIG. 2B), can also be formed with different verticalprofile shapes, as shown in other examples below.

More explicitly, a first emissive element 300 a has first thickness 302between its top 204 and bottom 206 surfaces and is associated with afirst wavelength of emissive element light emission. A second emissiveelement 300 b has second thickness 304 between its top 204 and bottom206 surfaces, less than the first thickness 302, and is associated witha second wavelength of emissive element light emission. A first fluidicassembly key 306 a encapsulates, at least partially, the first emissiveelement top and bottom surfaces 204/206 to form an overall encapsulatedemissive element third thickness 308. A second fluidic assembly key 306b encapsulates, at least partially, the second emissive element top andbottom surfaces 204/206 to form the overall encapsulated emissiveelement third thickness 308. As described in more detail below, the useof uniformly thick encapsulated emissive elements simplifies fluidicassembly deposition processes by permitting the use of uniformly deepdisplay substrate trap sites.

In another aspect, emissive elements 300 c and 300 d have respectivevertical profiles 310 c and 310 d, and the corresponding fluidicassembly keys 306 c and 306 d have a differing vertical profiles 312 cand 312 d, where a vertical profile is defined from a vantage orthogonalto the emissive element sidewalls 208. For example, as shown in FIG. 3C,fluidic assembly key 306 c has a vertical profile slope 314 c formedbetween a fluidic assembly key bottom surface 316 c, aligned with theemissive element bottom surface 206, and a fluidic assembly key topsurface 318 c, aligned with the emissive element top surface 204. Thefluidic assembly key top surface width 320 c is greater than the fluidicassembly key bottom surface width 322 c. In FIG. 3D, fluidic assemblykey 306 d has a vertical profile slope formed between a fluidic assemblykey bottom surface 316 d, aligned with the emissive element bottomsurface 206, and a fluidic assembly key top surface 318 d aligned withthe emissive element top surface 204. The fluidic assembly key topsurface and bottom surface widths 320 d are equal in this example.

In one aspect, the emissive element top 204 and bottom 206 surfaces aresubstantially planar in a horizontal orientation, parallel to horizontalplane 216. The emissive element sidewalls 208 form a vertically orientedslope 324 d between an emissive element top surface width 326 d lessthan the emissive element bottom surface width 328 d, as shown in FIG.3D, or a vertically oriented slope (of zero degrees) between an emissiveelement top surface and bottom surface widths 328 c, as shown in FIG.3C.

In FIG. 3A the keel is depicted as a cylindrically shaped structure, butother shapes are also useful, as depicted in FIGS. 3B, 3C, and 3D. Forexample, as seen from a top view the keel may have a rectangular, oval,oblong, or multi-faceted polygon shape, or as seen in cross-section, thekeel may be tapered. In addition, although the keel 213 is shown locatedat the center of the emissive element bottom surface in FIG. 3A, in FIG.3B the keel 213 b is not centered. In FIG. 3C keel 213 c tapers narrowlyto a point as it extends from the bottom surface 206. In contrast, FIG.3D depicts keel 213 d tapering to a minimum width at the intersectionwith the bottom surface 206. In fluidic deposition the flow velocitydecreases to zero at the substrate top surface. So the working principleof the keel is to create a low torque if the keel is up (correctlyoriented) with the emissive element top surface against the substratetop surface, or a high torque if the keel is down (against the substratetop surface) because a large area of the emissive element is fartherfrom the substrate top surface in faster moving fluid. Typically, thekeel has a shape that creates a point of contact with the substrate topsurface that is distant from the emissive element bottom surface, butclose to the central axis of the emissive element. Thus, the keel has asmall cross section to minimize fluid force when up (the emissiveelement top surface is adjacent the substrate top surface), but thatalso tilts the emissive element sufficiently that it flips in the flowwhen incorrectly oriented (upside down).

FIGS. 4A and 4B are, respectively, partial cross-sectional and planviews of a fluidic assembly emissive display. The display 400 comprisesa substrate 402 with a top surface 404 and a plurality of trap sites406-0 through 406-n, where n is an integer greater than one. Someexamples of emissive displays include televisions, computer monitors,handheld device screens, and backlights for LCD displays, as might beused for some of the above-mentioned examples, or as a direct emissiondisplay. The n number of trap sites may represent a single pixel with nsubpixels (n emission colors). For simplicity, only a single pixel isshown. The trap sites 406-0 through 406-n, which may also be referred toas wells, cavities, apertures, or orifices, are formed in the substratetop surface 404, with each trap site having an opening 408 withrespective horizontal shapes 428-0 through 428-n, defined from a vantageorthogonal to the substrate top surface 404. Each trap site 406-0through 406-n also has a bottom surface 412 with at least a firstelectrical interface 414 connected to a corresponding intersection witha row or column in an underlying enablement matrix 417. As shown, thebottom surfaces 412 of trap sites 406-0 and 406-1 also include a secondelectrical interface 416.

The matrix or array can be powered as a passive matrix so each row isturned on in sequence with each sub-pixel in the array powered at acontrolled current to produce the required brightness. However, due tosampling and power restraints this simple driving scheme is necessarilylimited to a relatively small number of rows. Alternatively, eachsub-pixel can be controlled by a thin-film transistor (TFT) drivingcircuit (not shown), which can control the amount of drive current basedon the charge stored in a capacitor (not shown). This active matrix (AM)circuit configuration allows the uLED to be powered nearly 100% of thetime so there is no limit on the number of rows in a display, except forthe power supplied to each column. Additional details concerning thematrix can be found in U.S. Pat. No. 9,825,202, which is incorporatedherein by reference.

Encapsulated emissive elements 418-0 through 418-n respectively occupytrap sites 406-0 through 406-n. Encapsulated emissive elements 418-0through 418-n respectively comprise emissive elements 420-0 through420-n. As shown in FIGS. 2A and 2B, each emissive element has a profile,top surface, a bottom surface, and sidewall surfaces between the top andbottom surfaces. Emissive elements 420-0 and 420-n have first electricalcontacts 210 connected to corresponding trap site first electricalinterfaces 414. In addition, emissive elements 420-0 and 420-1 havesecond electrical contacts 212 connected to corresponding trap sitesecond electrical interfaces 416. In this example, emissive elements420-0 through 420-n also comprise a keel 213. Fluidic assembly keys422-0 through 422-n respectively, at least partially, encapsulate theemissive element to form a profile, different than the emissive elementprofile.

As described in FIGS. 2A through 2D, each emissive element top surface206 is substantially planar in a horizontal orientation, parallel toelectrode plane 216. As shown in FIG. 4B, emissive elements 420-0through 420-n have respective horizontal profiles 424-0 through 424-nand the fluidic assembly keys 422-0 through 422-n have differinghorizontal profiles 426-0 through 426-n, as defined from a vantageorthogonal to the emissive element top surface 204. The displaysubstrate top surface 404 is substantially planar in the horizontalorientation, parallel to electrode plane 216, and trap site openingshapes 428-0 through 428-n respectively match the horizontal profile ofits occupying encapsulated emissive element fluidic assembly key.

As mentioned above, the wavelength of light emitted by a particularemissive element may be associated with a corresponding fluidic assemblykey horizontal profile. More explicitly, assuming that n=2, the displaymay comprise a first horizontally oriented shape 428-0, matching fluidicassembly key profile 426-0, a second horizontally oriented shape 428-1,differing from the first horizontally oriented shape but matchingprofile 426-1, and a third horizontally oriented shape 428-n, differingfrom the first and second horizontally oriented shapes, but matchingprofile 426-n. In this context, “matching” means being able to acceptone particular type of profile while rejecting other types of profiles.Each horizontally oriented shape 428-0 through 428-n may then beassociated with a different wavelength of emissive element lightemission. Likewise, as described in more detail in the explanation ofFIGS. 2A through 2D, differing horizontal or vertical shapes (as definedby thickness or sidewall slope) may also be associated with a differentwavelength of emissive element light emission.

In one aspect, emissive element 420-0 has a first thickness between itstop and bottom surfaces and is associated with a first wavelength ofemissive element light emission, while emissive element 420-1 has asecond thickness between its top and bottom surfaces, less than thefirst thickness, and is associated with a second wavelength of emissiveelement light emission. Fluidic assembly key 422-0 encapsulates theemissive element 420-0 top and bottom surfaces to form an overallencapsulated emissive element third thickness, and fluidic assembly key422-1 encapsulates the emissive element 420-1 top and bottom surfaces toform the overall encapsulated emissive element third thickness. In theinterest of simplifying FIG. 4A, details of the particular surfaces andthicknesses are presented in the description of the encapsulatedemissive elements of FIGS. 3A and 3B. Since the thicknesses of theencapsulated emissive elements 418-0 and 418-1 are the same, trap sites406-0 and 406-1 can be made to the same depth to simplify substratefabrication.

As noted in the explanation of FIGS. 3C and 3D, emissive elements andtheir corresponding fluidic assembly keys may have differing verticalprofiles, where a vertical profile is defined from a vantage orthogonalto the emissive element sidewalls. In FIG. 4A, the fluidic assembly keys422-0 and 422-1 have a vertical profile slope formed between a fluidicassembly key bottom surface, aligned with the emissive element bottomsurface, and a fluidic assembly key top surface aligned with theemissive element top surface. The fluidic assembly key top surface widthis greater than the fluidic assembly key bottom surface width. Detailsof a fluidic assembly key having a greater top surface width can befound in the description of FIG. 3D above. Alternatively, fluidicassembly key 422-n has equal top and bottom widths, as shown in greaterdetail in FIG. 3C.

As shown in FIG. 4A, substrate trap sites 406-0 through 406-11 comprisesidewalls with a vertical profile slope formed between the trap siteopening 408 and the trap site bottom surface 412. For trap sites 406-0and 406-1, the width 430 of the substrate trap site bottom surface 412is less than the width 432 of the trap site opening 408. Alternatively,as shown in trap site 406-n, the width of the substrate trap site bottomsurface 412 is equal to the width of the trap site opening 408.

Typically, the emissive element top and bottom surfaces aresubstantially planar in a horizontal orientation and the emissiveelement sidewalls form a vertically oriented slope between an emissiveelement top surface width less than the emissive element bottom surfacewidth (see FIG. 3D), or equal to an emissive element bottom surfacewidth (see FIG. 3C).

The fluidic assembly composite microcomponents may be either vertical orflip-chip configuration micro-LED devices that are at least partiallyenclosed, for example, in a polymeric matrix that is not contiguous withother micro-LEDs. In other words, individual micro-LEDs are processed tomake this protective polymer structure prior to harvest into suspension.The harvest into suspension is a critical step because high-speed,low-cost assembly is necessarily massively parallel. The fluidicassembly of micro-LED suspensions is the most developed parallelapproach in high-speed micro-LED assembly, but the stochastic nature ofthe process exposes the micro-LEDs to collisions, shear, and generalstresses that are less well controlled than deterministic assemblyapproaches such as pick-and-place. Here, the mechanical stressesexperienced by micro-LEDs during harvest from the growth or handlingsubstrates, and agitation of the assembly suspension, occurs betweendevices that are protected by the polymer structure.

Efficient assembly is sensitive to the shape and size of both thecomponent and trap site on the substrate—forming the component in such away that the assembly characteristics are dominated by the shape andcharacteristics of the polymer allows optimization of the assemblycharacteristics independent of the height, shape, angle, edge, and sizetolerance limitations of micro-LED processing. Gallium nitride (GaN) isa difficult material to pattern so the dry etching process using highenergy chlorine radicals tends to produce a tapered sidewall as themasking material is eroded. The masking material may be a photoresist ora hard mask such as silicon dioxide (SiO₂), chromium, or nickel. Theresulting micro-LED may have a sidewall with an inward taper that can befrom 60 to 80 degrees, where 90 degrees is vertical. It is verydifficult to produce a sidewall slope greater than about 75 degrees.Because of the nature of the GaN patterning process it is prohibitive toproduce a reentrant sidewall shape (greater than 90 degrees) on themicro-LED device itself. In the case of fluidic assembly a reentrantsidewall shape is formed as a result of the LED top surface width beinggreater than the bottom surface width. Fortunately, a negative actingphotopolymer can be patterned to produce a reentrant sidewall profile bycontrol of the exposure dose and development time of thephotolithography process.

FIGS. 5A through 5C are partial cross-sectional views depictingexemplary emissive element and fluidic assembly key sidewall profilescontrasting the component shape to the trap site shape. FIG. 5A shows atypical micro-LED shape with a sidewall 208 which is tapered inward dueto the low selectivity of the GaN isolation etch. It can be seen thatthe shape of the micro-LED sidewall dictates that the contact point 500between the micro-LED 502 and the trap site sidewall 504 is near the topof the trap site. In contrast, the polymer encapsulant 506 with avertical sidewall 508 can contact 500 a vertical trap site sidewall 504over most of the trap site height as shown in FIG. 5B. In FIG. 5C, theencapsulant 506 with an outward taper can contact 500 the trap sitesidewall 504 near the bottom. The position of the contact point 500between trap site and micro-LED determines the rotational moment inducedby fluid flow, which in turn determines the detrapping probability for acaptured micro-LED.

Additionally, outward-tapering encapsulant sidewalls provide a surfacewhere purely lateral forces can result in detrap of captured micro-LEDs.It is also important to note that the most advantageous component andtrap site shapes for high-speed fluidic assembly are generally notexactly complementary. The dynamics of trapping and detrapping areasymmetric and the encapsulant described herein provides a structurethat can be readily optimized for assembly efficiency and displayquality.

Lastly, the large-scale and stochastic nature of fluidic assembly mustaccount for uncertainty in trap site shapes, micro-LED shapes, and howthe micro-LED is disposed in the trap site. Micro-LEDs may often tiltand move in trap sites during assembly and the encapsulant shapesdescribed herein enable tuning the forces of those interactions in anenhanced manner.

FIGS. 6A and 6B are, respectively, partial cross-sectional and plan (topdown) views of a micro-LED with contacts in the vertical chipconfiguration. Also shown are a first semiconductor layer 604, MQW layer606, and second semiconductor layer 608. The first and second conductorlayers may be either n or p doped, but they are of opposite polarity.For simplicity, the figures and descriptions are of a micro-LED with thefirst semiconductor layer 604 being n doped. A via 600 in theencapsulant 602 allows an electrical contact (anode) 210 to themicro-LED top surface 204, while the cathode contact 212 is made to aregion unpassivated by the encapsulant. As can be seen, the encapsulantsize and shape are independent of the micro-LED size and shape, and thepolymer encapsulant 602 also leaves at least one plane or surface of themicro-LED uncovered. As the encapsulant 602 may be opaque or reflectiveto prevent light transmission, this opening may be utilized for lightextraction from the micro-LED as well as for the cathode contact 212shown. The anode contact 210, while shown with a smaller diameter thanthe micro-LED, could extend to a larger diameter than the micro-LEDwidth, particularly, in the case where light is intended to be directedthrough the bottom surface of the device.

FIGS. 7A through 7I are partial cross-sectional views depicting a sampleprocess flow that enables the partial encapsulation of a verticalconfiguration micro-LED in a patterned polymer matrix. In FIG. 7A themicro-LED heterostructure 700, such as variably doped gallium nitride,is epitaxially grown on a sapphire substrate 702 and covered with ametal stack and/or transparent conductor like indium tin oxide (ITO) 703for electrical contact to the device anode. For example, theheterostructure 700 may comprise a p-GaN layer 704, and MQW layer 706,and n-GaN layer 708. In FIG. 7B the micro-LEDs are etched to formindividual emitters. In FIG. 7C the isolated micro-LEDs are then coatedwith the patternable polymer matrix 710. One useful material is SU8which is spun on and photopatterned, but other material choices includealternate photoresists or non-photopatternable polymers, which areetched in FIG. 7D to form patterns after deposition and curing. Thepattern for a partially encapsulated vertical configuration micro-LED isthe isolation trench 712 around the micro-LED with the desired size,shape, sidewall profile, etc. and a via opening 714 through to themicro-LED's top surface. In FIG. 7E the via is metallized to form anelectrical contact 716 to the top surface of the micro-LED. Thismetallization may be done via plating, patterned metal deposition, orclip-coating, which may be planarized after deposition. In FIG. 7F theisolated devices are bonded to a temporary handling matrix (handlingsubstrate) 718. Typically, an adhesive or wax 719 is used to attach thehandle substrate to the isolated devices. In the interest of brevity theadhesive/wax layer is shown in the FIGS. 8, 10, and 11 series ofdrawings, but not labeled with a reference designator. In FIG. 7G themicrocomponents are then released from the growth substrate 702 with anappropriate process, such as laser lift-off for GaN devices or anappropriate wet etch for gallium arsenide (GaAs) based devices such asred LEDs fabricated from aluminum gallium indium phosphide (AlGaInP).After laser lift-off the growth substrate is removed leaving the LEDspositioned on the carrier (handling) substrate with the bottom surfacesexposed. A metal electrode 720 suitable for electrical contact to thedevice cathode is deposited and patterned. After the electrode iscompleted, an optional keel structure 722 may be formed in FIG. 7H thatis useful to provide orientation control in fluidic assembly. In FIG. 7Ithe temporary handling matrix has been dissolved.

FIGS. 8A through 8J are partial cross-sectional views depicting analternative sample process flow that enables the partial encapsulationof a vertical configuration micro-LED in a patterned polymer matrixusing a thin buffer layer between encapsulant and growth substrate. Apotential issue with the process flow of FIGS. 7A through 7I is that theencapsulant polymer is in direct contact with the growth substrate. Forrelease of the component using, for example, laser lift-off, it may bedesirable to leave a continuous interface between the epitaxial layerand the growth substrate. The steps associated with FIGS. 8A through 8G,8I, and 8J are essentially identical to the steps associated with FIGS.7A through 7I, except the etch of the micro-LED heterostructure 700 inFIG. 8B is stopped before reaching the growth substrate 702 and theprocess continues as shown. FIG. 8H depicts the results of etching ofthe remaining thin GaN 800 (see FIG. 8G), left after the etchingperformed in FIG. 8B.

FIGS. 9A and 9B are, respectively, partial cross-sectional and planviews showing encapsulation prior to release from a growth substrate.FIGS. 9C and 9D are, respectively, partial cross-sectional and planviews showing encapsulation after release from a growth substrate. FIG.9E is a plan view showing partial encapsulation of the emissive element.While vertical LEDs have several significant advantages for thefabrication of emissive displays, they do require additional processingafter fluidic assembly to make the top electrical interface. To simplifyprocessing after fluidic assembly an emissive display may utilizeflip-chip or surface mount configurations where both LED contacts are onthe top surface so they can be aligned with electrodes in the substrateand are bonded through a standard method such as solder reflow. In thisconfiguration, cathode 904 and anode 906 electrodes extend from the topsurface of the micro-LED 900 as shown in two possible configurations.The difference between the two structures is whether the encapsulant 902is applied before (see FIGS. 10A through 10I) or after release from theepitaxial growth substrate (see FIGS. 11A through 11H). In contrast toFIGS. 9B and 9D, which show the horizontal profile of the encapsulant902 completely extending beyond the horizontal profile of the emissiveelement 900, in FIG. 9E the encapsulated emissive element has an overallhorizontal profile that is partially the result of the emissive elementhorizontal profile and partially the result of the fluidic assembly keyhorizontal profile.

FIGS. 10A through 10I depict an exemplary process flow for the partialencapsulation of surface mount configuration composite microcomponentsprior to release from a growth substrate. Although not explicitly shown,the same processing option utilizing thin GaN and described in FIGS. 8Band 8H can be used in this process flow. In FIG. 10A the micro-LEDheterostructure 700, such as variably doped gallium nitride, isepitaxially grown on a sapphire substrate 702. The top surface may becoated with a metal stack and/or a transparent conductor like ITO 703for electrical contact to the device anode and cathode. In FIG. 10B themicro-LED heterostructure emitter areas are etched as a mesa, andsimultaneously etched to isolate and form individual micro-LEDs. In FIG.10C the isolated micro-LEDs are then coated with the patternable polymermatrix, which is etched in FIG. 10D to form patterns after depositionand curing. The pattern for a partially encapsulated flip-chipconfiguration micro-LED includes via openings 1000 and 1001 through tothe micro-LED's top surface. In FIG. 10E the vias are metallized to formelectrical contacts 1002 and 1003 to the top surface of the micro-LED.In FIG. 10F the isolated devices are bonded to a temporary handlingmatrix. In FIG. 10G the microcomponents are then released from thegrowth substrate 702 with an appropriate process, such as laser lift-offfor GaN devices or an appropriate wet etch for GaAs based devices. Afterlaser lift-off the growth substrate is removed leaving the LEDspositioned on the handling substrate 718 with the bottom surfacesexposed. An optional keel structure 722 may be formed in FIG. 1011 thatis useful to provide orientation control in fluidic assembly. In FIG.10I the temporary handling matrix has been dissolved.

FIGS. 11A through 11H depict an exemplary process flow for the partialencapsulation of surface mount configuration composite microcomponentsafter release from a growth substrate. Although not explicitly shown,the same processing option utilizing thin GaN and described in FIGS. 8Band 8H can be used in this process flow. In FIG. 11A the micro-LEDheterostructure 700, such as variably doped gallium nitride, isepitaxially grown on a sapphire substrate 702. The top surface may becoated with a metal stack and/or a transparent conductor like ITO 703for electrical contact to the device anode and cathode. In FIG. 11B themicro-LED heterostructure emitter areas are etched as a mesa, andsimultaneously etched to isolate and form individual micro-LEDs. In FIG.11C electrodes 1002 and 1003 are formed. In FIG. 11D the devices arebonded to a handling substrate 718 and in FIG. 11E the devices areseparated from the growth substrate 702. In FIG. 11F backside etching isoptionally performed and the micro-LEDs are then coated with thepatternable polymer matrix 710, which is etched. In FIG. 11G a keel 722is optionally formed on the encapsulated LED bottom surface, and in FIG.11H the handling substrate has been dissolved.

FIGS. 12A and 12B are partial cross-sectional views depicting a displaysubstrate before and after the deposition of encapsulated verticalemissive elements. The substrate 402 includes an array of trap siteswith an electrode 416 at the bottom of the trap site. The trap site isconfigured to accommodate the composite microcomponent in an orientationthat aligns the microcomponent electrode 210 with the substrateelectrode 416. The trap sites are co-optimized for assembly with thepartially encapsulated micro-LED. The cathode 212 may be contactedthrough additional metal patterning after infill, which is not shown. Asnoted above, the encapsulated emissive elements and trap sites may havematching horizontal profile shapes.

FIGS. 13A and 13B are partial cross-sectional views depicting a displaysubstrate before and after the deposition of encapsulated surface mountemissive elements. In this aspect there are two electrical interfaces414 and 416 in the bottom of each trap site configured to match theelectrodes 210 and 212 on the flip-chip micro-LEDs. As noted above, theencapsulated emissive elements and trap sites may have matchinghorizontal profile shapes.

FIG. 14 is a schematic depicting the relationship between trap siteshapes, encapsulant shapes, and emissive element shapes. In addition tothe enhanced control of composite microcomponent assemblycharacteristics through the patterned encapsulant, a major benefit ofthe encapsulant is the increased fracture resistance that reducesdefects and also allows for the use of more complicated shapes thatwould be more susceptible to fracture were the microcomponent to be abare die. While circular disk shaped microcomponents can be captured inany orientation by circular wells, simultaneous parallel assembly ofdistinct microcomponents such as red, green, and blue emitters, requiresmutually exclusive trapping. I.e., red emitters must only be allowed totrap in sites intended for red emission and be excluded from green orblue emitting sites; and similarly for blue and green emitters. As canbe seen, triangular, square, and circular encapsulated emissive elementshapes with matching trap site shapes, permit shape-selectivesimultaneous fluidic assembly.

In some cases, however, the microcomponents are only excluded frommismatched wells by relatively small areas with comparatively lessstructural strength (such as the triangular microcomponent over thecircular well). Preserving the structural integrity of themicrocomponent in these cases is enhanced by an excluding portionpossessing sufficient mechanical strength. In this manner, fragilemicro-LEDs may be assembled with shape-selective simultaneous fluidicassembly with fewer defects.

While FIG. 14 depicts the micro-LEDs inside each encapsulant shell asbeing identically sized, it can be seen that the use of the encapsulantpermits the size and shape of the micro-LED to be varied independentlyfrom the size and shape of the encapsulant. Thus, it is possible tooptimize emitter areas for chromaticity in the case of simultaneous red,green, and blue emitting micro-LED assembly while maintaining theoptimum device geometry for fluidic assembly. Likewise, the use of anencapsulant enables the use of different thickness micro-LEDs, resultingin composite microcomponents with identical thicknesses—which is asubstantial advantage in facilely producing emissive substrates usingfluidic assembly.

FIGS. 15A and 15B are partial cross-sectional views of a surface mountemissive element showing the use of an encapsulant as an electricalinsulator. A polymer encapsulation can also be used to enhance thefunction of micro-LEDs in other ways, depending on the detailedproperties of the material used. Photo patternable polymers areelectrical insulators. So coating the LED top surface preventselectrical leakage current between cathode and anode electrodes bypassivating surface damage resulting from the etch process.

FIGS. 16A through 16C are partial cross-sectional views depicting theuse of an encapsulating material in light management. It is alsopossible to add light management components to the polymer encapsulantto improve the performance of the micro-LED display. One of the mostimportant factors for improving image quality is to prevent light fromescaping from a pixel to contaminate an adjacent pixel, as schematicallydepicted in FIG. 16A. The substrate and well structures used for fluidicassembly are typically transparent so some of the light emitted by amicro-LED can be injected into these structures. Light injected into thesubstrate can travel a long distance laterally due to total internalreflection (TIR) before it is scattered. The undesirable result is thatlight from one pixel can emerge from an adjacent pixel, effectivelycontaminating the image of the adjacent pixel. In order to prevent thiseffect, the polymer encapsulant can be modified to absorb or reflectlight as respectively shown in FIGS. 16B and 16C. For absorbing light,the polymer additives might be a simple absorber like carbon black orgraphene oxide or it could be a more complex molecule such as a dyeselected to have an absorption edge at a specific wavelength. Thepolymer additive selected to reflect light could be titanium oxide(TiO₂) or silver nanoparticles. Although not explicitly shown, theencapsulation material may include additional components to contributefunctionality, such as magnetism, scattering, light extraction,wavelength conversion, or stiction. It should also be understood that invarious aspects the fluidic assembly key may act to planarize emissiveelement top and bottom surfaces to minimize surface area effect duringfluidic assembly.

FIG. 17 is a flowchart illustrating a method for fabricating anencapsulated emissive element. Although the method is depicted as asequence of numbered steps for clarity, the numbering does notnecessarily dictate the order of the steps. It should be understood thatsome of these steps may be skipped, performed in parallel, or performedwithout the requirement of maintaining a strict order of sequence.Generally however, the method follows the numeric order of the depictedsteps. The method starts at Step 1700.

Step 1702 provides a growth substrate. Step 1704 forms a plurality ofemissive elements. Each emissive element comprises a bottom surfaceattached to a top surface of the growth substrate, a top surface, and aprofile. Step 1706 conformally coats the growth substrate top surfacewith an encapsulation material. As noted above, the encapsulant may be aphotoresist, a polymer, a light reflective, magnetic, or a lightabsorbing material. Step 1708 patterns the encapsulation material toform fluidic assembly keys having a profile differing from the emissiveelement profiles.

In one aspect, as described in the explanation of FIGS. 7A-7I, 8A-8J,and 10A-10I, patterning the encapsulation material in Step 1708 includesforming a contact opening to each emissive element top surface. Then,Step 1710 forms a first electrical contact to each emissive element topsurface. In the case of a surface mount emissive element, Step 1708forms two contact openings and Step 1710 additionally forms a secondelectrical contact. Step 1712 bonds the emissive element top surfaces toa handling substrate. Step 1714 separates the emissive elements from thegrowth substrate, and in Step 1716 the emissive elements are separatedfrom the handling substrate.

In another aspect, the difference in process flow is depicted usingdotted connecting lines, and is as described in the explanation of FIGS.11A through 11H. In this aspect, prior to conformally coating the growthsubstrate top surface with the encapsulation material, Step 1705 a formsa first electrical contact to each emissive element top surface. Step1705 b bonds the emissive element top surfaces to a handling substrate,and Step 1705 c separates the emissive elements from the growthsubstrate.

Using either method, Step 1715 optionally forms one or more fluidicassembly keels on each emissive element bottom surface before separatingthe emissive elements from the handling substrate in Step 1716.

In one aspect, forming the emissive element top surfaces in Step 1704includes forming emissive element top surfaces substantially planar in ahorizontal orientation, and with a horizontal profile. Then, forming thefluidic assembly key profile in Step 1708 includes forming a fluidicassembly key horizontal profile differing from the emissive elementhorizontal profile, as defined from a vantage orthogonal to the emissiveelement top surface. For example, Step 1704 may form emissive elementscapable of emitting light having a first wavelength, and Step 1708 mayform a first fluidic assembly key horizontal profile corresponding tothe first wavelength. In addition, Step 1704 may form emissive elementhorizontal profiles from a plurality of differing horizontally orientedshapes, where each horizontally oriented shape is associated with adifferent wavelength of emissive element light emission. Even if theemissive element profiles are necessarily different, Step 1704 may forma plurality of emissive element types (not necessarily simultaneously),with each type capable of emitting light at a different wavelength.Then, Step 1708 forms fluidic assembly key profiles from a plurality ofdifferent horizontally oriented shapes, with each horizontally orientedshape associated with a corresponding emissive element wavelength oflight emission. Alternatively, Step 1708 forms fluidic assembly keyshapes with a plurality of different vertically oriented shapes, witheach vertically oriented shape associated with a corresponding emissiveelement wavelength of light emission. As noted above, a horizontalprofile is defined from a vantage orthogonal to the emissive element topsurface and a vertical profile is defined from the vantage orthogonal toan emissive element sidewall.

In addition to the fluidic assembly keys optionally being formed withdifferent vertical profiles, Step 1704 may form emissive elements havinga vertical profile, and Step 1708 may form fluidic assembly key verticalprofiles differing from the emissive element vertical profiles. Forexample, Step 1708 may form fluidic assembly keys with a verticalprofile having a slope formed between the fluidic assembly key bottomsurface, which is aligned with the emissive element bottom surface, andthe fluidic assembly key top surface aligned with the emissive elementtop surface. The fluidic assembly key top surface width may be greaterthan or equal to the fluidic assembly key bottom surface width.Likewise, the emissive elements formed in Step 1704 may have top andbottom surfaces that are substantially planar in a horizontalorientation, with emissive element sidewalls forming a verticallyoriented slope between an emissive element top surface width less thanor equal to than an emissive element bottom surface width.

Thus, as noted above, Step 1704 may form emissive elements fromdiffering horizontal or vertically oriented shapes, where each differentemissive element shape is associated with a different wavelength oflight emission. In one aspect, the vertically oriented shapes aredefined by a thickness between the emissive element top and bottomsurfaces. Thus, Step 1704 may form first emissive elements having afirst thickness between its top and bottom surfaces, associated with afirst wavelength of emissive element light emission. Step 1704 may alsoform second emissive elements, not necessarily as the first emissiveelements are being formed, having a second thickness between its top andbottom surfaces, less than the first thickness, associated with a secondwavelength of emissive element light emission. Then, Step 1708 formsfirst fluidic assembly key encapsulating the first emissive element topand bottom surfaces to form an overall encapsulated emissive elementthird thickness. Step 1708 also forms second fluidic assembly keysencapsulating the second emissive element top and bottom surfaces toform the overall encapsulated emissive element third thickness.

An encapsulated emissive element, a display fabricated usingencapsulated emissive elements, and an associated encapsulated emissiveelement fabrication method have been provided. Examples of particularmaterials, dimensions, profiles, and circuit layouts have been presentedto illustrate the invention. Although emissive elements, particularlyLEDs, have been presented, the methods described herein are alsoapplicable to other two-terminal devices such as photodiodes,thermistors, pressure sensors, and piezoelectric devices, which may alsobe encapsulated. Other variations and embodiments of the invention willoccur to those skilled in the art.

We claim:
 1. An encapsulated emissive element comprising: a micro-lightemitting diode (micro-LED), with a maximum cross-section of 100 micronsor less, having a profile selected from a group consisting of differinghorizontal profile shapes, as defined from a plan, top-down vieworthogonal to the micro-LED top surface and sidewall surfaces, ordiffering vertical thicknesses between the micro-LED top and bottomsurfaces, and comprising a top surface, a bottom surface, sidewallsurfaces between the top and bottom surfaces, and a pair of electricalcontacts, and wherein each micro-LED profile is associated with adifferent wavelength of emissive element light emission; a fluidicassembly key exposing the micro-LED electrical contacts while at leastpartially encapsulating the micro-LED to form a profile, different thanthe micro-LED profile, made from a material selected from the groupconsisting of polymer, photoresist, light reflective, magnetic, andlight absorbing materials; wherein a first micro-LED has first thicknessbetween its top and bottom surfaces and is associated with a firstwavelength of micro-LED light emission; wherein a second micro-LED hassecond thickness between its top and bottom surfaces, less than thefirst thickness, and is associated with a second wavelength of micro-LEDlight emission; wherein a first fluidic assembly key encapsulates thefirst micro-LED top and bottom surfaces to form an overall encapsulatedemissive element third thickness; and, wherein a second fluidic assemblykey encapsulates the second micro-LED top and bottom surfaces to formthe overall encapsulated emissive element third thickness.
 2. Theencapsulated emissive element of claim 1 wherein the micro-LED has avertical profile and the fluidic assembly key has a differing verticalprofile, where a vertical profile is defined from a vantage orthogonalto the micro-LED sidewalls and micro-LED top surface.
 3. Theencapsulated emissive element of claim 2 wherein the fluidic assemblykey has a vertical profile slope formed between a fluidic assembly keybottom surface, aligned with the micro-LED bottom surface, and a fluidicassembly key top surface aligned with the micro-LED top surface; and,wherein the fluidic assembly key top surface width is greater than orequal to the fluidic assembly key bottom surface width.
 4. Theencapsulated emissive element of claim 3 wherein the micro-LED top andbottom surfaces are substantially planar in a horizontal orientation;and, wherein the micro-LED sidewalls form a vertically oriented slopebetween a micro-LED top surface width less than or equal to a micro-LEDbottom surface width.
 5. The encapsulated emissive element of claim 1wherein the micro-LED further comprises a keel extending from themicro-LED bottom surface.
 6. The encapsulated emissive element of claim1 wherein the micro-LED electrical contacts are arranged in anorientation selected from the group consisting of vertical, with a firstelectrical contact mounted on the micro-LED top surface and a secondelectrical contact mounted on the micro-LED bottom surface, and surfacemount, with both the first and second electrical contacts mounted on themicro-LED top surface.
 7. The encapsulated emissive element of claim 1wherein the micro-LED is a surface mount micro-LED with a first and asecond electrical contact mounted on micro-LED top surface; and, whereinthe fluidic assembly key is an electrical insulator isolating the firstelectrical contact from the second electrical contact.
 8. A fluidicassembly emissive display comprising: a substrate comprising: a topsurface; a plurality of trap sites formed in the substrate top surface,with each trap site having sidewalls and an opening with a horizontalshape, defined from a view orthogonal to the substrate top surface andtrap site sidewalls, and a bottom surface with at least a firstelectrical interface connected to a corresponding intersection in anunderlying row/column enablement matrix; an encapsulated emissiveelement occupying each trap site, each encapsulated emissive elementcomprising: a micro-light emitting diode (micro-LED), with a maximumcross-section of 100 microns or less, comprising a profile, a topsurface, a bottom surface, sidewall surfaces between the top and bottomsurfaces, a first electrical contact mounted on the micro-LED topsurface and connected to a corresponding trap site first electricalinterface, and a second electrical contact; a fluidic assembly keyexposing the micro-LED electrical contacts while at least partiallyencapsulating the micro-LED to form a profile, different than themicro-LED profile, made from a material selected from the groupconsisting of polymer, photoresist, light reflective, magnetic, andlight absorbing material; wherein a first micro-LED type has firstthickness between its top and bottom surfaces and is associated with afirst wavelength of micro-LED light emission; wherein a second micro-LEDtype has second thickness between its top and bottom surfaces, less thanthe first thickness, and is associated with a second wavelength ofmicro-LED light emission; wherein a first fluidic assembly key typeencapsulates the first micro-LED top and bottom surfaces to form anoverall encapsulated emissive element third thickness; and, wherein asecond fluidic assembly key type encapsulates the second micro-LED topand bottom surfaces to form the overall encapsulated emissive elementthird thickness.
 9. The fluidic assembly emissive display of claim 8wherein each micro-LED top surface is substantially planar in ahorizontal orientation; wherein each micro-LED has a horizontal profileand the fluidic assembly key has a differing horizontal profile, asdefined from a plan, top-down view orthogonal to the micro-LED topsurface and sidewall surfaces; and, wherein the display substrate topsurface is substantially planar in the horizontal orientation and eachtrap site opening shape matches the horizontal profile of its occupyingencapsulated emissive element fluidic assembly key.
 10. The fluidicassembly display of claim 8 wherein each micro-LED has a verticalprofile and the fluidic assembly key has a differing vertical profile,where a vertical profile is defined from a view orthogonal to themicro-LED sidewalls and micro-LED top surface.
 11. The fluidic assemblydisplay of claim 10 wherein the fluidic assembly keys have a verticalprofile slope formed between a fluidic assembly key bottom surface,aligned with the micro-LED bottom surface, and a fluidic assembly keytop surface aligned with the micro-LED top surface; and, wherein thefluidic assembly key top surface width is greater than or equal to thefluidic assembly key bottom surface width.
 12. The fluidic assemblydisplay of claim 11 wherein the substrate trap sites comprise sidewallswith a vertical profile slope formed between the trap site opening andthe trap site bottom surface; and, wherein a width of the substrate trapsite bottom surface is less than or equal to a width of the trap siteopening.
 13. The fluidic assembly display of claim 11 wherein themicro-LED top and bottom surfaces are substantially planar in ahorizontal orientation; and, wherein the micro-LED sidewalls form avertically oriented slope between a micro-LED top surface width lessthan or equal to a micro-LED bottom surface width.
 14. The fluidicassembly display of claim 8 wherein each micro-LED further comprises akeel extending from the micro-LED bottom surface.
 15. The fluidicassembly display of claim 8 wherein the micro-LED electrical contactsare arranged in an orientation selected from the group consisting ofvertical, with a second electrical contact mounted on the micro-LEDbottom surface, and surface mount, with the second electrical contactmounted on the micro-LED top surface and connected to a secondelectrical interface mounted on the trap site bottom surface.
 16. Thefluidic assembly display of claim 8 wherein the micro-LEDs are surfacemount emissive elements with a first and a second electrical contactmounted to the micro-LED top surface; and, wherein the fluidic assemblykeys are an electrical insulator isolating the first electrical contactfrom the second electrical contact.
 17. An encapsulated emissive elementcomprising: a micro-light emitting diode (micro-LED), with a maximumcross-section of 100 microns or less, having a horizontal profile andcomprising a top surface, a bottom surface, sidewall surfaces betweenthe top and bottom surfaces, and a pair of electrical contacts, where ahorizontal profile is defined from a plan, top-down view orthogonal tothe micro-LED top surface and sidewall surfaces; a fluidic assembly keyat least partially encapsulating the micro-LED to form a horizontalprofile, different than the micro-LED horizontal profile, the fluidicassembly key having top surface and a bottom surface; wherein thefluidic assembly key bottom surface is formed in a same horizontal planeas the micro-LED bottom surface, and the fluidic assembly key topsurface overlies the micro-LED top surface with at least one openingexposing the micro-LED electrical contacts; wherein the micro-LEDprofile also includes differing vertical thicknesses between themicro-LED top and bottom surfaces; and, wherein each micro-LED profileis associated with a different wavelength of micro-LED light emission.18. The encapsulated emissive element of claim 17 wherein the micro-LEDis capable of emitting light having a first wavelength, and wherein thefirst wavelength is associated with a first fluidic assembly keyhorizontal profile.
 19. The encapsulated emissive element of claim 18wherein the fluidic assembly key horizontal profile is selected from aplurality of different horizontally oriented shapes; and, wherein eachhorizontally oriented shape is associated with a different wavelength ofmicro-LED light emission.
 20. The encapsulated emissive element of claim17 wherein a first micro-LED has first thickness between its top andbottom surfaces and is associated with a first wavelength of micro-LEDlight emission; wherein a second micro-LED has second thickness betweenits top and bottom surfaces, less than the first thickness, and isassociated with a second wavelength of micro-LED light emission; whereina first fluidic assembly key encapsulates the first micro-LED top andbottom surfaces to form an overall encapsulated emissive element thirdthickness; and, wherein a second fluidic assembly key encapsulates thesecond micro-LED top and bottom surfaces to form the overallencapsulated emissive element third thickness.
 21. The encapsulatedemissive element of claim 17 wherein the micro-LED has a verticalprofile and the fluidic assembly key has a differing vertical profile,where a vertical profile is defined from a view orthogonal to themicro-LED sidewalls and micro-LED top surface.
 22. The encapsulatedemissive element of claim 21 wherein the fluidic assembly key has avertical profile slope formed between a fluidic assembly key bottomsurface, aligned with the micro-LED bottom surface, and a fluidicassembly key top surface aligned with the micro-LED top surface; and,wherein the fluidic assembly key top surface width is greater than orequal to the fluidic assembly key bottom surface width.
 23. Theencapsulated emissive element of claim 22 wherein the micro-LED top andbottom surfaces are substantially planar in a horizontal orientation;and, wherein the micro-LED sidewalls form a vertically oriented slopebetween a micro-LED top surface width less than or equal to a micro-LEDbottom surface width.
 24. The encapsulated emissive element of claim 17wherein the micro-LED further comprises a keel extending from themicro-LED bottom surface.
 25. The encapsulated emissive element of claim17 wherein the micro-LED electrical contacts are arranged in anorientation selected from the group consisting of vertical, with a firstelectrical contact mounted on the micro-LED top surface and a secondelectrical contact mounted on the micro-LED bottom surface, and surfacemount, with both the first and second electrical contacts mounted on themicro-LED top surface.
 26. The encapsulated emissive element of claim 17wherein the micro-LED is a surface mount emissive element with a firstand a second electrical contact mounted on the micro-LED top surface;and, wherein the fluidic assembly key is an electrical insulatorisolating the first electrical contact from the second electricalcontact.
 27. An encapsulated emissive element comprising: a micro-lightemitting diode (micro-LED), with a maximum cross-section of 100 micronsor less, having a profile and comprising a top surface, a bottomsurface, a keel extending from the bottom surface, sidewall surfacesbetween the top and bottom surfaces, and a pair of electrical contacts;and, a fluidic assembly key exposing the micro-LED electrical contactswhile at least partially encapsulating the micro-LED to form a profile,different than the micro-LED profile, made from a material selected fromthe group consisting of polymer, photoresist, light reflective,magnetic, and light absorbing materials.
 28. The encapsulated emissiveelement of claim 27 wherein the micro-LED top surface is substantiallyplanar in a horizontal orientation; and, wherein the micro-LED has ahorizontal profile and the fluidic assembly key has a differinghorizontal profile, as defined from a plan, top-down view orthogonal tothe micro-LED top surface and sidewall surfaces.
 29. The encapsulatedemissive element of claim 28 wherein the micro-LED is capable ofemitting light having a first wavelength, and wherein the firstwavelength is associated with a first fluidic assembly key horizontalprofile.
 30. The encapsulated emissive element of claim 29 wherein thefluidic assembly key horizontal profile is selected from a plurality ofdifferent horizontally oriented shapes; and, wherein each horizontallyoriented shape is associated with a different wavelength of micro-LEDlight emission.
 31. The encapsulated emissive element of claim 27wherein the micro-LED profile is selected from a group consisting ofdiffering horizontal profile shapes, as defined from a plan, top-downview orthogonal to the micro-LED top surface and sidewall surfaces, ordiffering vertical thicknesses between the micro-LED top and bottomsurfaces; and, wherein each micro-LED profile is associated with adifferent wavelength of emissive element light emission.
 32. Theencapsulated emissive element of claim 27 wherein the micro-LED has avertical profile and the fluidic assembly key has a differing verticalprofile, where a vertical profile is defined from a vantage orthogonalto the micro-LED sidewalls and micro-LED top surface.
 33. Theencapsulated emissive element of claim 32 wherein the fluidic assemblykey has a vertical profile slope formed between a fluidic assembly keybottom surface, aligned with the micro-LED bottom surface, and a fluidicassembly key top surface aligned with the micro-LED top surface; and,wherein the fluidic assembly key top surface width is greater than orequal to the fluidic assembly key bottom surface width.
 34. A fluidicassembly emissive display comprising: a substrate comprising: a topsurface; a plurality of trap sites formed in the substrate top surface,with each trap site having sidewalls and an opening with a horizontalshape, defined from a view orthogonal to the substrate top surface andtrap site sidewalls, and a bottom surface with at least a firstelectrical interface connected to a corresponding intersection in anunderlying row/column enablement matrix; an encapsulated emissiveelement occupying each trap site, each encapsulated emissive elementcomprising: a micro-light emitting diode (micro-LED), with a maximumcross-section of 100 microns or less, comprising a profile, a topsurface, a bottom surface, a keel extending from the bottom surface,sidewall surfaces between the top and bottom surfaces, a firstelectrical contact mounted on the micro-LED top surface and connected toa corresponding trap site first electrical interface, and a secondelectrical contact; and, a fluidic assembly key exposing the micro-LEDelectrical contacts while at least partially encapsulating the micro-LEDto form a profile, different than the micro-LED profile, made from amaterial selected from the group consisting of polymer, photoresist,light reflective, magnetic, and light absorbing material.
 35. A fluidicassembly emissive display comprising: a substrate comprising: a topsurface; a plurality of trap sites formed in the substrate top surface,with each trap site having sidewalls and an opening with a horizontalshape, defined from a view orthogonal to the substrate top surface andtrap site sidewalls, and a bottom surface with at least a firstelectrical interface connected to a corresponding intersection in anunderlying row/column enablement matrix; an encapsulated emissiveelement occupying each trap site, each encapsulated emissive elementcomprising: a micro-light emitting diode (micro-LED), with a maximumcross-section of 100 microns or less, comprising a profile, a topsurface, a bottom surface, sidewall surfaces between the top and bottomsurfaces, a first electrical contact mounted on the micro-LED topsurface and connected to a corresponding trap site first electricalinterface, and a second electrical contact; a fluidic assembly keyexposing the micro-LED electrical contacts while at least partiallyencapsulating the micro-LED to form a profile, different than themicro-LED profile, made from a material selected from the groupconsisting of polymer, photoresist, light reflective, magnetic, andlight absorbing material; wherein each micro-LED has a vertical profileand the fluidic assembly key has a differing vertical profile, where avertical profile is defined from a view orthogonal to the micro-LEDsidewalls and micro-LED top surface; wherein the fluidic assembly keyshave a vertical profile slope formed between a fluidic assembly keybottom surface, aligned with the micro-LED bottom surface, and a fluidicassembly key top surface aligned with the micro-LED top surface; and,wherein the fluidic assembly key top surface width is greater than orequal to the fluidic assembly key bottom surface width.
 36. The fluidicassembly display of claim 35 wherein the substrate trap sites comprisesidewalls with a vertical profile slope formed between the trap siteopening and the trap site bottom surface; and, wherein a width of thesubstrate trap site bottom surface is less than or equal to a width ofthe trap site opening.
 37. The fluidic assembly display of claim 35wherein the micro-LED top and bottom surfaces are substantially planarin a horizontal orientation; and, wherein the micro-LED sidewalls form avertically oriented slope between a micro-LED top surface width lessthan or equal to a micro-LED bottom surface width.
 38. An encapsulatedemissive element comprising: a micro-light emitting diode (micro-LED),with a maximum cross-section of 100 microns or less, having a horizontalprofile and comprising a top surface, a bottom surface, a keel extendingfrom the bottom surface, sidewall surfaces between the top and bottomsurfaces, and a pair of electrical contacts, where a horizontal profileis defined from a plan, top-down view orthogonal to the micro-LED topsurface and sidewall surfaces; a fluidic assembly key at least partiallyencapsulating the micro-LED to form a horizontal profile, different thanthe micro-LED horizontal profile, the fluidic assembly key having topsurface and a bottom surface; and, wherein the fluidic assembly keybottom surface is formed in a same horizontal plane as the micro-LEDbottom surface, and the fluidic assembly key top surface overlies themicro-LED top surface with at least one opening exposing the micro-LEDelectrical contacts.